Switching power supplies have been used for several decades to improve system efficiency and reduce size and weight. In particular, AC/DC converters which take in AC utility power and output isolated low (typically) voltage DC for use by system circuits are largely replacing their linear counterparts. The traditional linear supplies are composed of a transformer which operates at the AC line frequency, and output rectifier/filter circuit, and a linear regulator to maintain a consistent output voltage. Switching AC/DC converters rectify and filter the incoming AC line to create a high-voltage DC bus. A chopper circuit drives a high frequency transformer with pulses of this DC bus at frequencies typically in the 10's or 100's of kilohertz.
The unfortunate side affect of the switching action is the creation of electromagnetic interference (EMI). EMI can be both radiated and conducted (out the power cord for example) and can interfere with other nearby electronics. For this reason, various government and pseudo-government agencies around the world specify limitations on the allowable amount of EMI. To address the conducted EMI, power supply designers typically add an L-C (inductor-capacitor) EMI filter to the AC line interface. The filter is typically designed to reduce both differential mode (Line-Line) and common mode (Line-Ground) noise.
For AC/DC converters as well as other electronics, surges from lightning and AC power grid operations are a field hazard. Vast numbers of electronics are destroyed every year from surges. As with EMI, various government and pseudo-government agencies around the world specify minimum surge immunity levels which depend on the type of product and installation practices for that product. Both common mode and differential mode surge requirements are typically specified. Compliance is normally verified by testing with a surge generator that simulates lightning induced surges. Various surge waveforms are used for different applications. For AC line surges, a waveform with a 1.2 us rise time and an exponential decay with a 50 us half power decay time (1.2×50 us) is typically used.
Many electronic products are now double insulated and use only a two wire AC power cord. As such, common mode surge requirements are not applicable for many of these devices as there is no ground connection. However, certain devices such as a cable modem, an embedded multimedia terminal adapter (“eMTA”), a cable set-top box, a television, etc are affected. These devices get connected to ground through the coax cable connection. This becomes a further liability for these devices because the coax ground often has a less than optimal connection to the power ground of the installation. This can lead to elevated surge levels than would otherwise be experienced. As a result, these types of devices are often damaged from common mode surges. To cope with this, many cable operators specify surge immunity requirements in excess of the standards. For example, the standard for an eMTA is 2 kV (1.2×50 us waveform) common mode, but some cable operators require up to 10 kV.
As shown in FIG. 1, power supply 2 uses a common mode choke 4 together with a common mode capacitor 6 that spans the boundary between the primary and the secondary sections of the power supply. Inductor 4 and capacitor 6 operate as a filter to filter EMI from the switching action of the power supply, thus preventing the EMI energy from emanating onto the AC power network from power supply 2.
These components, including the common mode choke 4 and common mode capacitor 6 are typically capable of withstanding voltages of 2 KV, and even 10 KV without being damaged. However, when a voltage surge is applied to the inputs of power supply 2, an oscillation, or resonant ringing, often occurs in the L-C circuit formed by common mode choke 4 and common mode capacitor 6. This causes the instantaneous voltage stress to be greater than the peak of the actual applied surge voltage.
A surge test signal applied by surge generator 7 to the inputs of a power supply 2 as shown in FIG. 1 is shown by trace 8 in FIG. 2. The resulting resonant ringing across the common mode capacitor 6 is shown by trace 10. In practice, non-linearities such as common mode choke saturation would typically limit, but not eliminate this ringing. It is noted that some of the oscillation caused by ringing, as shown by peaks 12 and 14 in FIG. 2 exceed the input signal, some by a factor of 1.5 or more. This high amplitude resonant ringing is impressed upon every component that spans the isolation boundary between the primary and secondary side of the power supply (i.e., common mode capacitor 6, transformer 15 and the feedback circuit). Although the common mode capacitor 6 and transformer 15 shown in FIG. 1 may be capable of sustaining 10 KV energy levels, first oscillation peak 12 shown in FIG. 2 may exceed the surge voltage that these components are able to withstand. As discussed above, a surge signal represented by trace 8 may be impressed during testing by a common mode surge generator, or the surge/spike may be impressed by a nearby lightning strike when a device is deployed in the field.
In a device that is designed for use in an electrical system where ground 16 is connected to a reliable earth ground with either a three wire cord or a dedicated ground wire, one or more Metal Oxide Varistors (“MOV”) may be used across the line and neutral inputs, across the line input and ground 16 and across neutral and ground 16.
However, some providers require that a device be operable even when a reliable ground 16 is not available. This may be because the local electrical distribution system does not provide a reliable ground, because the building where the device is to be used does not provide a reliable ground, or because the building where the device is to be used does not provide three-prong electrical receptacles. In such scenarios, the MOVs would not provide adequate protection because ‘ground’ node 16 could float, thus not providing an low potential ‘destination’ for charge that would otherwise be shunted by the MOVs around common mode capacitor 6. Furthermore, safety regulations typically do not allow the connection of surge dissipative devices such as MOVs to a ‘functional ground’ such as the coax cable shield.
Thus, there is a need in the art for a system for protecting power supply circuitry in an electrical device from high voltage surges when a reliable earth ground is not available for the device's circuitry.